The proposed research deals with the synthesis, isolation, characterization, and physicochemical study of low molecular weight complexes of iron with sulfur-containing ligands. The objects of synthesis are complexes containing FenSn core (n equals 2,4,8) additionally stabilized by mono- or polyfunctional mercaptide ligands, including cysteinyl polypeptides. These complexes are to be designed in a manner such that they have the potential of simulating to a significant degree the structures (where known) and electronic properties of iron-sulfur proteins, commonly called the ferredoxins. Suitable crystalline materials whose stoichiometries and spectroscopic features are similar to those of protein active sites will be subjected to x-ray studies in order to establish structures, which may serve as guides for those of the protein active sites. These and other synthetic complexes, including those interrelated by electron transfer reactions, will be examined in detail by nmr, epr, optical, Mossbauer, and photoelectron spectroscopy and by electrochemical methods. These results will be related to the extensive body of similar information for the proteins in order to contribute to the elucidation of their unique biophysical characteristics, which in many instances have resisted satisfactory interpretation due to uncertainties in composition and structure of active sites.